1. Field of the Invention
The present invention relates to a circuit connection structure which connects a flat cable to a printed circuit board, and a printed circuit board.
2. Description of the Related Art
Usually, a flat cable is connected to a printed circuit board through a connector. However, because a connector has usually a plug-in allowance of a prescribed amount, there are cases where a worker who performs part replacement erroneously inserts a flat cable obliquely to a connector as shown in FIG. 7. In FIG. 7, reference numeral 101 denotes a printed circuit board, reference numeral 110 denotes a printed wiring board, reference numeral 102 denotes a flat cable, and reference numeral 103 denotes a connector formed on the printed wiring board 110. Reference numerals 111 and 121 denote power supply lines and reference numerals 113 and 122 denote ground lines.
Recent years have seen miniature designs and thin designs of electronic equipment and, for this reason, the terminal-to-terminal gap of a flexible flat cable (FFC) has provided a narrow pitch of 0.5 mm or less. Therefore, if the flat cable 102 is obliquely inserted, the adjacent lines 121 and 122 formed in the flat cable 102 come into erroneous contact with each other through a contact 131 of the connector 103, whereby there is a fear that electronic equipment may be damaged.
Hence as disclosed by Japanese Patent Application Laid-Open No. H09-289064, there has been considered a method of detecting insertions by using a flat cable having a special terminal construction in which each terminal length at both ends is short. Furthermore, as shown in FIG. 8, means for preventing erroneous contact between lines may sometimes be taken by setting a gap between a power supply line 221 and an adjacent line 222 in a flat cable 202 at not less than twice a usual gap as simpler and inexpensive means. According to this method, it is possible to prevent damage to equipment at low cost.
However, the method disclosed by Japanese Patent Application Laid-Open No. H09-289064 requires an additional step of shape addition for a usual flat cable. Furthermore, because it is necessary to provide a circuit for detecting oblique insertions, the cost rises greatly.
The method shown in FIG. 8 had the problem that if the spacing from a ground line adjacent to the power supply line widens, the value of radiation noise becomes high. That is, if a high-frequency noise current generated in a digital circuit flows through a flat cable, the flat cable works as an unintended antenna and radiation noise is generated. The value of radiation noise is proportional to a high-frequency noise current and the size of a loop area of a return channel of the high-frequency noise current. A high-frequency noise current which flows in the power supply line uses the adjacent ground line having a low impedance as its return channel. The narrower the gap of the power supply line to the return channel, the smaller the loop area of a high-frequency noise current and, therefore, the value of radiation noise becomes smaller. Conversely, the wider the gap between the power supply line and the return channel, the larger the loop area of a high-frequency noise current, and hence the value of radiation noise also becomes larger.